Time-of-Use Electricity Cost Calculator (Peak, Shoulder, Off-Peak)

This calculator totals electricity cost across peak, shoulder, and off-peak periods. Example: 300 kWh peak at 30 cents, 200 kWh shoulder at 20 cents, and 500 kWh off-peak at 10 cents costs USD 160 before any taxes or surcharges.

This calculation is used during operating-cost review for electrically intensive loads such as HVAC equipment, electric water heating, battery charging, process equipment, EV charging, and large branch-circuit or feeder loads. It helps quantify whether moving controllable kWh from a high-cost period to an off-peak period changes the expected utility cost.

The calculation does not determine conductor ampacity, AWG or kcmil conductor size, raceway fill, overcurrent protection, voltage drop, or service capacity. Those are separate electrical design and code decisions. A lower utility rate may support an operational decision to schedule a load differently, but it does not reduce the actual current, continuous-load requirements, terminal limitations, or equipment rating required when that load operates.

Billing-Period Energy Inputs

Enter energy use and the corresponding energy price for each available billing period:

Input	Unit	Electrical billing purpose
Peak kWh	kWh	Energy used during the utility’s highest-priced period
Peak rate	$/kWh	Energy charge assigned to peak-period use
Shoulder kWh	kWh	Energy used during the intermediate-priced period
Shoulder rate	$/kWh	Energy charge assigned to shoulder-period use
Off-peak kWh	kWh	Energy used during the lowest-priced period
Off-peak rate	$/kWh	Energy charge assigned to off-peak use
Fixed charge	$	Flat monthly charge added after the energy charges

A kWh is an energy quantity, not a measure of instantaneous electrical load. For example, a 10 kW load operating for 3 hours consumes 30 kWh, subject to actual equipment demand and operating conditions. The same 30 kWh can have different costs when used in different time windows.

The period definitions must match the electric utility’s tariff. Peak, shoulder, and off-peak schedules can vary by season, weekday, holiday schedule, service class, and rate plan.

Period Cost Calculation

Each period cost is calculated by multiplying energy consumption by that period’s stated rate:

\(\displaystyle \text{Peak cost} = \text{Peak kWh} \times \text{Peak rate}\)

\(\displaystyle \text{Shoulder cost} = \text{Shoulder kWh} \times \text{Shoulder rate}\)

\(\displaystyle \text{Off-peak cost} = \text{Off-peak kWh} \times \text{Off-peak rate}\)

The calculator then combines the three energy charges:

\(\displaystyle \text{Energy cost} = \text{Peak cost} + \text{Shoulder cost} + \text{Off-peak cost}\)

Total energy consumption is:

\(\displaystyle \text{Total kWh} = \text{Peak kWh} + \text{Shoulder kWh} + \text{Off-peak kWh}\)

The estimated utility bill adds the flat charge:

\(\displaystyle \text{Total bill} = \text{Energy cost} + \text{Fixed charge}\)

The Effective rate expresses the entire calculated bill, including the fixed charge, as an average cost per kWh:

\(\displaystyle \text{Effective rate} = \frac{\text{Total bill}}{\text{Total kWh}}\)

This blended rate is useful for budget forecasting and comparing monthly operating patterns. It is not the same as any individual period rate.

Calculation Example

Using the following period energy use and rates:

Field	Value
Peak kWh	300 kWh
Peak rate	$0.25/kWh
Shoulder kWh	200 kWh
Shoulder rate	$0.18/kWh
Off-peak kWh	500 kWh
Off-peak rate	$0.10/kWh
Fixed charge	$20

The period charges are:

\(\displaystyle \text{Peak cost} = 300 \times 0.25 = $75\)

\(\displaystyle \text{Shoulder cost} = 200 \times 0.18 = $36\)

\(\displaystyle \text{Off-peak cost} = 500 \times 0.10 = $50\)

\(\displaystyle \text{Energy cost} = 75 + 36 + 50 = $161\)

\(\displaystyle \text{Total kWh} = 300 + 200 + 500 = 1{,}000\text{ kWh}\)

\(\displaystyle \text{Total bill} = 161 + 20 = $181\)

\(\displaystyle \text{Effective rate} = \frac{181}{1{,}000} = $0.181/\text{kWh}\)

The calculated results are Peak cost: $75, Shoulder cost: $36, Off-peak cost: $50, Energy cost: $161, Total kWh: 1,000 kWh, Total bill: $181, and Effective rate: $0.181/kWh.

Electrical Load-Management Use

Time-of-use billing is most useful when a load can be shifted without changing the electrical installation. A battery-energy-storage system, EV charging equipment, thermal storage, or scheduled process load may consume the same kWh while producing a different monthly energy charge.

For example, shifting 100 kWh from the stated peak period to the stated off-peak period changes the energy charge by:

\(\displaystyle 100 \times ($0.25 - $0.10) = $15\)

The kWh is unchanged, but the estimated energy cost falls by $15. The calculated savings does not alter branch-circuit ampacity, feeder ampacity, voltage-drop performance, disconnect rating, panel capacity, or the load’s maximum operating current.

Where load scheduling changes the simultaneous operation of equipment, the electrical design review may need separate attention. A change in coincident load can affect service and feeder load calculations, generator loading, transformer loading, voltage-drop conditions, and demand-management strategy. Those determinations require the actual equipment characteristics, operating sequence, applicable electrical code provisions, and AHJ requirements.

Tariff and Field Limits

The calculation applies only the entered period kWh, period rates, and Fixed charge. It does not include demand charges, taxes, riders, credits, tier resets, billing adjustments, minimum bills, net-metering treatment, special tariff provisions, or the timing rules used by a specific utility statement.

Verify the utility rate schedule, meter data, billing dates, seasonal definitions, and service classification before using the result for a financial decision. Use the issued utility bill and governing tariff to reconcile actual charges.